1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to demagnetizing the head in a disk drive by increasing the frequency of an AC write signal while maintaining the write current amplitude.
2. Description of the Prior Art
FIG. 1 is a prior art recording format showing a plurality of embedded servo sectors 20–2N recorded on the disk 4 of a disk drive which define a plurality of radially-spaced, concentric data tracks 6. Each data track 6 is partitioned into a plurality of data sectors wherein the servo sectors 20–2N are considered “embedded” in the data sectors. Each servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a servo data field 12 comprising coarse head positioning information such as a Gray coded track addresses for use during seeking, and servo bursts 14 which provide fine head positioning information for use during tracking. In a “headerless” format, the servo data field 12 also includes information identifying the data sectors between the servo sectors.
User data is written to a data sector by modulating the write current applied to a head 16, which magnetizes the surface of the disk 4. With conventional longitudinal magnetic recording, the head 16 magnetizes a thin film layer in a planar direction and in the more recent perpendicular magnet recording, the head 16 magnetizes the thin film layer in a perpendicular direction. In either case, the head 16 may remain magnetized after a write operation creating a magnetic field that can erase data in the following data sectors. A prior art technique (such as disclosed in U.S. Pat. No. 5,168,395) teaches to demagnetize the head 16 after every write operation by applying an AC write signal to the head 16 while gradually decreasing the write current amplitude. This technique is illustrated in FIGS. 2A–2C wherein FIG. 2A shows the write data applied to the write circuitry, including an AC write signal at the end of the write operation. The write current amplitude is gradually decreased as shown in FIG. 2B, which decreases the amplitude of the AC, write signal when observed at the head 16 as shown in FIG. 2C. As the amplitude of the AC write signal decreases the head 16 is demagnetized. A problem with this technique, however, is the difficulty and expense involved with controlling the write current amplitude at the end of every write operation. In addition, the AC write signal must be applied over a sufficient interval to guarantee demagnetization which requires a corresponding “gap” at the end of each data sector. This gap is undesirable since it decreases the amount of disk space available for storing user data (i.e., decreases format efficiency).
There is, therefore, a need to simplify head demagnetization in a disk drive in a manner that improves format efficiency.